1. Field of the Invention
The present invention relates to an operational amplifier, and more particularly to a high-speed low-drift operational amplifier having a frequency band of 0 to 100 MHz and a temperature coefficient of offset voltage of about 1.2 .mu.V/.degree.C.
2. Description of the Prior Art
A chopper-type circuit such as shown in FIG. 1 of the accompanying drawings has been widely employed as a high-speed low-drift operational amplifier. Referring to FIG. 1, a high frequency component of an input signal e.sub.i applied to an input terminal 1 is amplified by a high-speed operational amplifier 6, and a low frequency component of the input signal is amplified by a low-drift amplifier 9 which is made up of a chopper circuit 2, an a.c. amplifier 3, a synchronous rectifier 4 and a smoothing circuit 5. Thus, the circuit shown in FIG. 1 can produce, on the whole, an output signal e.sub.o having a high-speed low-drift characteristic, which is delivered from an output terminal 7. In the low-drift amplifier 9, the following operations are performed. The input signal e.sub.i is converted by the chopper circuit 2 into a square wave signal, namely, an a.c. signal, which is amplified by the amplifier 3 and then detected by the snychronous rectifier 4. The square wave signal delivered from the rectifier 4 is smoothed by the smoothing circuit 5. Respective switches of the chopper circuit 2 and the synchronous rectifier 4 are supplied with an on-off signal which is generated by a square wave generator 8. In the circuit shown in FIG. 1, the chopper noise resulting from the make and break of the switch which is effected by the output of the square wave generator 8, has frequently come into question. Further, the above circuit is complicated in construction, and cannot perform a differential operation in the input thereof.